@ASLR: I would point out that oceans are globally still absorbing CO2, not emitting. You can say that El Nino slows this absorption, but it does not trigger net emission (apart from fires of course).
First, I note that it is well known that the primary source of CO₂ fluctuations over the ENSO cycle is due to changes in land vegetation in the tropics (from 30N to 30S), rather than due to emissions from the ocean. Second, the first reference (and associated image) shows that there has been a two-fold increase of carbon cycle sensitivity to tropical temperature variations over the past several decades. Third, the second reference indicates global warming is increasing the frequency of extreme El Ninos. As strong El Ninos increase both the temperature and induce droughts in the tropics it is clear that CO₂ emissions increase from the tropical land vegetation during strong El Ninos:
Wang, X., Piao, S., Ciais, P., Friedlingstein, P., Myneni, R.B., Cox, P., Heimann, M., Miller, J., Peng, S.P., Wang, T., Yang, H. and Chen, A., (2014), "A two-fold increase of carbon cycle sensitivity to tropical temperature variations", Nature, 2014; DOI: 10.1038/nature12915.
http://www.nature.com/nature/journal/v506/n7487/full/nature12915.html#extended-datahttp://sites.bu.edu/cliveg/files/2014/01/wang-nature-2014.pdfAbstract: "Earth system models project that the tropical land carbon sink will decrease in size in response to an increase in warming and drought during this century, probably causing a positive climate feedback. But available data are too limited at present to test the predicted changes in the tropical carbon balance in response to climate change. Long-term atmospheric carbon dioxide data provide a global record that integrates the interannual variability of the global carbon balance. Multiple lines of evidence demonstrate that most of this variability originates in the terrestrial biosphere. In particular, the year-to-year variations in the atmospheric carbon dioxide growth rate (CGR) are thought to be the result of fluctuations in the carbon fluxes of tropical land areas. Recently, the response of CGR to tropical climate interannual variability was used to put a constraint on the sensitivity of tropical land carbon to climate change. Here we use the long-term CGR record from Mauna Loa and the South Pole to show that the sensitivity of CGR to tropical temperature interannual variability has increased by a factor of 1.9 ± 0.3 in the past five decades. We find that this sensitivity was greater when tropical land regions experienced drier conditions. This suggests that the sensitivity of CGR to interannual temperature variations is regulated by moisture conditions, even though the direct correlation between CGR and tropical precipitation is weak. We also find that present terrestrial carbon cycle models do not capture the observed enhancement in CGR sensitivity in the past five decades. More realistic model predictions of future carbon cycle and climate feedbacks require a better understanding of the processes driving the response of tropical ecosystems to drought and warming."
Caption for image: " Figure 1 | Change in detrended anomalies in CGR and tropical MAT, in
dCGR/dMAT and in ªintCGR over the past five decades. a, Change in detrended CGR anomalies at Mauna Loa Observatory (black) and in detrended tropical MAT anomalies (red) derived from the CRU data set16. Tropical MAT is calculated as the spatial average over vegetated tropical lands (23uN to 23u S). The highest correlations between detrended CGR and detrended tropicalMAT are obtained when no time lags are applied (R50.53, P,0.01). b, Change in dCGR/dMAT during the past five decades. c, Change in cintCGR during the past five decades. In b and c, different colours showdCGR/dMATor cint CGR estimated with moving time windows of different lengths (20 yr and 25 yr). Years on the horizontal axis indicate the central year of the moving time window used to derive dCGR/dMAT or cintCGR (for example, 1970 represents period 1960–1979 in the 20-yr time window). The shaded areas show the confidence interval of dCGR/dMATand cintCGR, as appropriate, derived using 20-yr or 25-yr moving windows in 500 bootstrap estimates."
Wenju Cai, Agus Santoso, Guojian Wang, Sang-Wook Yeh, Soon-Il An, Kim M. Cobb, Mat Collins, Eric Guilyardi, Fei-Fei Jin, Jong-Seong Kug, Matthieu Lengaigne, Michael J. McPhaden, Ken Takahashi, Axel Timmermann, Gabriel Vecchi, Masahiro Watanabe & Lixin Wu (2015), "ENSO and greenhouse warming", Nature Climate Change, Volume: 5, Pages: 849–859, doi:10.1038/nclimate2743
http://www.nature.com/nclimate/journal/v5/n9/full/nclimate2743.htmlAbstract: "The El Niño/Southern Oscillation (ENSO) is the dominant climate phenomenon affecting extreme weather conditions worldwide. Its response to greenhouse warming has challenged scientists for decades, despite model agreement on projected changes in mean state. Recent studies have provided new insights into the elusive links between changes in ENSO and in the mean state of the Pacific climate. The projected slow-down in Walker circulation is expected to weaken equatorial Pacific Ocean currents, boosting the occurrences of eastward-propagating warm surface anomalies that characterize observed extreme El Niño events. Accelerated equatorial Pacific warming, particularly in the east, is expected to induce extreme rainfall in the eastern equatorial Pacific and extreme equatorward swings of the Pacific convergence zones, both of which are features of extreme El Niño. The frequency of extreme La Niña is also expected to increase in response to more extreme El Niños, an accelerated maritime continent warming and surface-intensified ocean warming. ENSO-related catastrophic weather events are thus likely to occur more frequently with unabated greenhouse-gas emissions. But model biases and recent observed strengthening of the Walker circulation highlight the need for further testing as new models, observations and insights become available."
See also, for input from Peter Cox:
http://www.nature.com/nature/journal/v494/n7437/full/nature11882.htmlExtract: "We estimate that over tropical land from latitude 30° north to 30° south, warming alone will release 53 ± 17 gigatonnes of carbon per kelvin."